GMAT PREP (Arithmetic Sequence)

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relaxin99: Senior | Next Rank: 100 Posts; Posts: 73; Joined: Tue Sep 16, 2008 4:33 pm; Thanked: 2 times

by relaxin99 » Mon Feb 16, 2009 4:36 pm

In the arithmetic sequence t1, t2, t3......tn....., t1 = 23... tn = tn-1 - 3 for each n>1. What is the value of n when tn= -4

A) -1
B) 7
C) 10
D) 14
E) 20

Can someone please explain their answer as well, the correct answer is down below

The answer is C

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Source: Beat The GMAT — Problem Solving | Mon Feb 16, 2009 4:36 pm

billzhao: Senior | Next Rank: 100 Posts; Posts: 80; Joined: Mon Feb 02, 2009 6:36 am; Thanked: 10 times

by billzhao » Mon Feb 16, 2009 4:42 pm

We know that tn=(tn-1)-3, so the sequence is with arithmetic progression with tolerance: d = -3.

We have tn=t1+(n-1)*d, where t1=23, tn=-4 and d=-3, so we can solve the equation and find n.

-4=23-3*(n-1) =>n=10

Yiliang

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tini: Junior | Next Rank: 30 Posts; Posts: 27; Joined: Sat Dec 29, 2007 1:37 amFollowed by: 1 member; Thanked: 2 times

by tini » Mon Feb 16, 2009 5:01 pm

tn can be expressed as tn=t1-(n-1)3

-4=23-(n-1)3
n-1=27/3=9
n=10

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